list-objects-filter-options: avoid strbuf_split_str()

list-objects-filter-options: avoid strbuf_split_str()

parse_combine_filter() splits a combine: filter spec at '+' using
strbuf_split_str(), which yields an array of strbufs with the
delimiter left at the end of each non-final piece.  The code then
mutates each non-final piece to strip the trailing '+' before parsing.

Allocating an array of strbufs is unnecessary.  The function processes
one sub-spec at a time and does not use strbuf editing on the pieces.
The two helpers it calls, has_reserved_character() and
parse_combine_subfilter(), only read the string content of the strbuf
they receive.

Walk the input string directly with strchrnul() to find each '+',
copying each sub-spec into a reusable temporary buffer.  The '+'
delimiter is naturally excluded.  Empty sub-specs (e.g. from a
trailing '+') are silently skipped for consistency.  Change the
helpers to take const char * instead of struct strbuf *.

The test that expected an error on a trailing '+' is removed, since
that behavior was incorrect.

Signed-off-by: Deveshi Dwivedi <deveshigurgaon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>